Priorities USA can thank three donors, who each kicked in $1 million, for its rise in funds:

Franklin L. Haney, founder of a real estate company that develops hotels and apartment buildings in Washington, D.C., may be Tennessee-born, but his ties to Washington are deep. It took two unsuccessful political bids — one for Congress in 1966, and then for Tennessee governor in 1974 — for Haney to realize he was better suited to fundraising than campaigning. He put aside his political ambitions and, through the success of his real estate business, has been a longtime Democratic fundraiser.

Barbara A. Stiefel, a philanthropist from Coral Gables, Fla., has been an active fundraiser for Florida Democrats.

And in Texas, trial lawyer Steve Mostyn is the state's largest Democratic donor this election cycle. This isn't the first time Mostyn has supported a political action committee. He was the financial backer for a much smaller PAC, Back to Basics, that was dedicated to defeating Gov. Rick Perry of Texas in 2010.

Restore Our Future also saw some new names popping up on its donor list.

Warren Stephens, an investment banker from Arkansas, gave a first-time donation of $500,000 to the superPAC.

Richard Mellon Scaife, owner of the Pittsburgh Tribune-Review and Newsmax, gave $67,500. Scaife is an heir to his great-uncle Andrew Mellon's fortune. He's directed much of his wealth to supporting conservative causes, and in the 1990s was one of former President Bill Clinton's harshest critics.

UnlikeScaife and Stephens, Rocco Ortenzio, a Pennsylvania health care executive, is one of Restore Our Future's repeat donors. The May filings bring his current total to $750,000 with a new donation of $500,000 just last month.

There were three separate donations of $333,333, $333,333 and $333,334 made by three different corporations in Dayton, Ohio, with matching return addresses. The Sunlight Foundation traced the million-dollar donation to the Reynolds and Reynolds Co. run by CEO Robert T. Brockman. Brockman has already given the maximum donation of $5,000 directly to Romney's campaign.

American Crossroads, another Republican superPAC, attracted two million-dollar donors last month, for a total haul of $4.6 million for May. The money will be used to help Romney and other GOP causes.